Method for etching phosphate ores

Abstract

A method for etching phosphate ores includes a single-pass digesting of ores which P 2 0 5 content is greater than 20% in weight by a hydrochloric aqueous acid solution having an HCI concentration less than 10% by weight with an etching solution formation and the separation of the insoluble solid phase and the aqueous phase of the etching solution. Preneutralization of the etching solution is accomplished by a neutralizing agent prior to the separation in such a way that the etching solution pH which is less than pH to which an important part of phosphate ions in solution precipitates in the form of calcium monohydrogen phosphate (DCP) is adjusted and in subsequently neutralizing the separated aqueous phase in such a way that a pure DCP is precipitated.

1. A method for etching phosphate ore, consisting of the sequential steps of:
 (a) a digestion, in one step and in co-current, of phosphate ore having a P 2 O 5  content of more than 20% by weight and an aqueous solution of hydrochloric acid having an HCI concentration of less than 10% by weight, which results in formation of an etching liquor consisting of an aqueous phase, in which calcium phosphate is in solution, and an insoluble solid phase which contains impurities, 
 (b) a preliminary neutralization of the etching liquor containing calcium phosphate in solution to a first pH which is between about 0.8 and 4.0, with precipitation of impurities, 
 (c) a first separation between the insoluble solid phase and the aqueous phase of the neutralized etching liquor, during which an isolation of the precipitated impurities takes place, 
 (d) a subsequent neutralization of the separated aqueous phase resulting from the first separation to a second pH which is greater than said first pH, with precipitation of calcium monohydrogen phosphate, and 
 (e) a second separation in this subsequently neutralized aqueous phase between an aqueous solution of calcium chloride and the precipitated DCP. 
 
     
     
       2. The method according to  claim 1 , comprising adjusting said second pH of said separated aqueous phase resulting from the first separation by said subsequent neutralization to a value of at least 4.5. 
     
     
       3. The method according  claim 1 , wherein said preliminary and subsequent neutralizations are carried out using a strong base selected from at least one of the group consisting of the hydroxide, the oxide and the water-soluble salts of calcium, sodium, potassium and ammonium. 
     
     
       4. The method according to  claim 1 , wherein the digestion is carried out at a temperature between about 20 and 80° C. 
     
     
       5. The method according to  claim 1 , further comprising a preliminary step of forming said aqueous solution of hydrochloric acid by diluting concentrated hydrochloric acid in water. 
     
     
       6. The method according to  claim 1 , further comprising a preliminary step of forming said aqueous solution of hydrochloric acid by treating an aqueous solution of calcium chloride with sulphuric acid and removing a calcium sulphate precipitate therefrom. 
     
     
       7. The method according to  claim 1 , wherein the phosphate ore has a P 2 O 5  content of 25 to 35% by weight. 
     
     
       8. The method according to  claim 1 , wherein said aqueous solution of hydrochloric acid which is used in the digestion has an HCI concentration of around 3 to 8% by weight. 
     
     
       9. The method according to  claim 1 , further comprising a treatment of said aqueous solution of calcium chloride with an aqueous solution of sulphuric acid, with formation of insoluble calcium sulphate, which precipitates, and of an aqueous phase based on hydrochloric acid, an isolation of the calcium sulphate precipitate, and an at least partial recycling, to the digestion step, of the aqueous phase based on hydrochloric acid, so as to form said aqueous solution of hydrochloric acid. 
     
     
       10. The method according to  claim 1 , further comprising an additional neutralization of said aqueous solution of calcium chloride, so as to adjust this aqueous solution to a pH which is greater than the pH of the subsequent neutralization and so as to precipitate residual impurities, and an elimination of these impurities from said aqueous solution, a treatment of the latter with an aqueous solution of sulphuric acid, with formation of insoluble calcium sulphate, which precipitates, and of an aqueous phase based on hydrochloric acid, an isolation of the calcium sulphate precipitate, and a recycling, to the digestion step, of the aqueous phase based on hydrochloric acid, so as to form said aqueous solution of hydrochloric acid. 
     
     
       11. The method according to  claim 10 , wherein the pH of said aqueous solution of calcium chloride is adjusted by said additional neutralization to a value of at least 8.5. 
     
     
       12. The method according to  claim 10 , wherein the additional neutralization is carried out using a strong base selected from at least one of the group consisting of the hydroxide, the oxide and the water-soluble salts of calcium, sodium, potassium and ammonium. 
     
     
       13. The method according to  claim 1 , wherein the digestion takes place in a reactor equipped with a stirrer.